DATABASE MANAGEMENT SYSTEM

CHAPTER 1.

INTRODUCTION

What is database?

The database is a collection of inter-related data which is used to retrieve, insert and delete the

data efficiently. It is also used to organize the data in the form of a table, schema, views, and

reports, etc.

For example: The college Database organizes the data about the admin, staff, students and faculty

etc.

Using the database, we can easily retrieve, insert, and delete the information.

What is data?

Data is a raw and unorganized fact that required to be processed to make it meaningful. Data can be

simple at the same time unorganized unless it is organized. Generally, data comprises facts, observations,

perceptions numbers, characters, symbols, image, etc.

What is information?

Information is a set of data which is processed in a meaningful way according to the given requirement.

Information assigns meaning and improves the reliability of the data.

Database Management System

o Database management system is a software which is used to manage the database. For example:

MySQL, Oracle, etc are a very popular commercial database which is used in different

applications.

o DBMS provides an interface to perform various operations like database creation, storing data in

it, updating data, creating a table in the database and a lot more.

DBMS allows users the following tasks:

o Data Definition: It is used for creation, modification, and removal of definition that defines the

organization of data in the database.

o Data Updation: It is used for the insertion, modification, and deletion of the actual data in the

database.

o Data Retrieval: It is used to retrieve the data from the database which can be used by applications

for various purposes.

o User Administration: It is used for registering and monitoring users, maintain data integrity,

enforcing data security, dealing with concurrency control, monitoring performance and recovering

information corrupted by unexpected failure.

DBMS vs. File System

DBMS FILE SYSTEM

DBMS is a collection of data. In DBMS, the

user is not required to write the procedures.

File system is a collection of data. In this

system, the user has to write the procedures

for managing the database.

DBMS gives an abstract view of data that

hides the details.

File system provides the detail of the data

representation and storage of data.

DBMS provides a crash recovery mechanism,

i.e., DBMS protects the user from the

system failure.

File system doesn't have a crash

mechanism, i.e., if the system crashes while

entering some data, then the content of the

file will lost.

DBMS provides a good protection

mechanism.

It is very difficult to protect a file under the

file system.

DBMS contains a wide variety of

sophisticated techniques to store and

retrieve the data.

File system can't efficiently store and

retrieve the data.

DBMS takes care of Concurrent access of

data using some form of locking.

In the File system, concurrent access has

many problems like redirecting the file while

other deleting some information or updating

some information.

Characteristics Of DBMS

o It uses a digital repository established on a server to store and manage the information.

o It can provide a clear and logical view of the process that manipulates data.

o DBMS contains automatic backup and recovery procedures.

o It can reduce the complex relationship between data.

o It is used to support manipulation and processing of data.

o It is used to provide security of data.

Advantages of DBMS

o Controls database redundancy: It can control data redundancy because it stores all the data in

one single database file and that recorded data is placed in the database.

o Data sharing: In DBMS, the authorized users of an organization can share the data among

multiple users.

o Easily Maintenance: It can be easily maintainable due to the centralized nature of the database

system.

o Reduce time: It reduces development time and maintenance need.

o Backup: It provides backup and recovery subsystems which create automatic backup of data from

hardware and software failures and restores the data if required.

o multiple user interface: It provides different types of user interfaces like graphical user

interfaces, application program interfaces

Disadvantages of DBMS

o Cost of Hardware and Software: It requires a high speed of data processor and large memory

size to run DBMS software.

o Size: It occupies a large space of disks and large memory to run them efficiently.

o Complexity: Database system creates additional complexity and requirements.

o Higher impact of failure: Failure is highly impacted the database because in most of the

organization, all the data stored in a single database and if the database is damaged due to electric

failure or database corruption then the data may be lost forever.

CHAPTER 2.

DATABASE ARCHITECTURE AND MODELING

The DBMS design depends upon its architecture. The basic client/server architecture is used to deal with

a large number of PCs, web servers, database servers and other components that are connected with

networks.

DBMS 3-tier Architecture

DBMS 3-tier architecture divides the complete system into three inter-related but independent

modules as shown below

1. Physical Level: At the physical level, the information about the location of database objects in the

data store is kept. Various users of DBMS are unaware of the locations of these objects.

2. Conceptual Level: At conceptual level, data is represented in the form of various database tables.

For Example, STUDENT database may contain STUDENT and COURSE tables which will be

visible to users but users are unaware of their storage.

3. External Level: An external level specifies a view of the data in terms of conceptual level tables.

Each external level view is used to cater to the needs of a particular category of users. For Example,

FACULTY of a university is interested in looking course details of students, STUDENTS are

interested in looking at all details related to academics, accounts, courses and hostel details as well.

So, different views can be generated for different users.

Data Independence

Data independence means a change of data at one level should not affect another level. Two types of data

independence are present in this architecture:

1. Physical Data Independence: Any change in the physical location of tables and indexes should not

affect the conceptual level or external view of data. This data independence is easy to achieve and

implemented by most of the DBMS.

2. Conceptual Data Independence: The data at conceptual level schema and external level schema

must be independent. This means a change in conceptual schema should not affect external schema.

e.g.; Adding or deleting attributes of a table should not affect the user’s view of the table. But this

type of independence is difficult to achieve as compared to physical data independence because the

changes in conceptual schema are reflected in the user’s view.

Database Design:

Phases of database design:

Database designing for a real-world application starts from capturing the requirements to physical

implementation using DBMS software which consists of following steps shown below:

Conceptual Design: The requirements of database are captured using high level conceptual data model.

For Example, the ER model is used for the conceptual design of the database.

Logical Design: Logical Design represents data in the form of relational model. ER diagram produced in

the conceptual design phase is used to convert the data into the Relational Model.

Physical Design: In physical design, data in relational model is implemented using commercial DBMS

like Oracle, DB2.

Chapter-3

ER Model

o ER model stands for an Entity-Relationship model. It is a high-level data model. This model is

used to define the data elements and relationship for a specified system.

o It develops a conceptual design for the database. It also develops a very simple and easy to design

view of data.

o In ER modeling, the database structure is portrayed as a diagram called an entity-relationship

diagram.

For example, Suppose we design a school database. In this database, the student will be an entity with

attributes like address, name, id, age, etc. The address can be another entity with attributes like city, street

name, pin code, etc and there will be a relationship between them.

Component Of ER Diagram

1. Entity:

An entity may be any object, class, person or place. In the ER diagram, an entity can be represented as

rectangles.

Consider an organization as an example- manager, product, employee, department etc. can be taken as an

entity.

a. Weak Entity

An entity that depends on another entity called a weak entity. The weak entity doesn't contain any key

attribute of its own. The weak entity is represented by a double rectangle.

2. Attribute

The attribute is used to describe the property of an entity. Eclipse is used to represent an attribute.

For example, id, age, contact number, name, etc. can be attributes of a student.

a. Key Attribute

The key attribute is used to represent the main characteristics of an entity. It represents a primary key. The

key attribute is represented by an ellipse with the text underlined.

b. Composite Attribute

An attribute that composed of many other attributes is known as a composite attribute. The composite

attribute is represented by an ellipse, and those ellipses are connected with an ellipse.

c. Multivalued Attribute

An attribute can have more than one value. These attributes are known as a multivalued attribute. The

double oval is used to represent multivalued attribute.

For example, a student can have more than one phone number.

d. Derived Attribute

An attribute that can be derived from other attribute is known as a derived attribute. It can be represented

by a dashed ellipse.

For example, A person's age changes over time and can be derived from another attribute like Date of

birth.

3. Relationship

A relationship is used to describe the relation between entities. Diamond or rhombus is used to represent

the relationship.

a. One-to-One Relationship

When only one instance of an entity is associated with the relationship, then it is known as one to one

relationship.

For example, A female can marry to one male, and a male can marry to one female.

b. One-to-many relationship

When only one instance of the entity on the left, and more than one instance of an entity on the right

associates with the relationship then this is known as a one-to-many relationship.

For example, Scientist can invent many inventions, but the invention is done by the only specific

scientist.

c. Many-to-one relationship

When more than one instance of the entity on the left, and only one instance of an entity on the right

associates with the relationship then it is known as a many-to-one relationship.

For example, Student enrolls for only one course, but a course can have many students.

d. Many-to-many relationship

When more than one instance of the entity on the left, and more than one instance of an entity on the right

associates with the relationship then it is known as a many-to-many relationship.

For example, Employee can assign by many projects and project can have many employees.

CONCEPT OF SUPERCLASS AND SUBCLASS

Superclass: An entity type that represents a general concept at a high level.

Subclass: An entity type that represents a specific concept at lower levels.

A subclass is said to inherit from a superclass. A subclass can inherit from many superclasses in the

hierarchy. When a subclass inherits from one or more superclasses, it inherits all their attributes. In

addition to the inherited attributes, a subclass can also define its own specific attributes. A subclass also

inherits participation in the relationship sets in which its superclass (higher-level entity) participates.

The process of making a superclass from a group of subclasses is called generalization. The process of

making subclasses from a general concept is called specialization.

Specialization: A means of identifying sub-groups within an entity set which have attributes that are not

shared by all the entities (top-down).

Specialization

o Specialization is a top-down approach, and it is opposite to Generalization. In specialization, one

higher level entity can be broken down into two lower level entities.

o Specialization is used to identify the subset of an entity set that shares some distinguishing

characteristics.

o Normally, the superclass is defined first, the subclass and its related attributes are defined next,

and relationship set are then added.

For example: In an Employee management system, EMPLOYEE entity can be specialized as TESTER

or DEVELOPER based on what role they play in the company.

Generalization: Multiple entity sets are synthesized into a higher-level entity set, based on common

features (bottom-up).

Generalization

o Generalization is like a bottom-up approach in which two or more entities of lower level combine

to form a higher level entity if they have some attributes in common.

o In generalization, an entity of a higher level can also combine with the entities of the lower level

to form a further higher level entity.

o Generalization is more like subclass and superclass system, but the only difference is the

approach. Generalization uses the bottom-up approach.

o In generalization, entities are combined to form a more generalized entity, i.e., subclasses are

combined to make a superclass.

For example, Faculty and Student entities can be generalized and create a higher level entity Person.

Aggregation

In aggregation, the relation between two entities is treated as a single entity. In aggregation, relationship

with its corresponding entities is aggregated into a higher level entity.

For example: Center entity offers the Course entity act as a single entity in the relationship which is in a

relationship with another entity visitor. In the real world, if a visitor visits a coaching center then he will

never enquiry about the Course only or just about the Center instead he will ask the enquiry about both.

Chapter 4.

What is RDBMS

RDBMS stands for Relational Database Management Systems.

Proposed by E.F. Codd

All modern database management systems like SQL, MS SQL Server, IBM DB2, ORACLE, My-SQL

and Microsoft Access are based on RDBMS.

Relational Data Base Management Systems (RDBMS) are database management systems that maintain

data records and indices in tables. Relationships may be created and maintained across and among the

data and tables. In a relational database, relationships between data items are expressed by means of

tables. This allows a high degree of data independence.

An RDBMS has the capability to recombine the data items from different files, providing powerful tools

for data usage.

How it works?

Data is represented in terms of tuples (rows) in RDBMS.

Relational database is most commonly used database. It contains number of tables and each table has its

own primary key.

Due to a collection of organized set of tables, data can be accessed easily in RDBMS.

What is table?

The RDBMS database uses tables to store data. A table is a collection of related data entries and contains

rows and columns to store data.

A table is the simplest example of data storage in RDBMS.

Let's see the example of student table.

What is field?

Field is a smaller entity of the table which contains specific information about every record in the table. In

the above example, the field in the student table consist of id, name, age, course.

What is row or record?

A row of a table is also called record. It contains the specific information of each individual entry in the

table. It is a horizontal entity in the table. For example: The above table contains 5 records.

Let's see one record/row in the table.

What is column?

A column is a vertical entity in the table which contains all information associated with a specific field in

a table. For example: "name" is a column in the above table which contains all information about student's

name.

NULL Values

The NULL value of the table specifies that the field has been left blank during record creation. It is totally

different from the value filled with zero or a field that contains space.

Data Integrity

There are the following categories of data integrity exist with each RDBMS:

Entity integrity: It specifies that there should be no duplicate rows in a table.

Domain integrity: It enforces valid entries for a given column by restricting the type, the format, or the

range of values.

Referential integrity: It specifies that rows cannot be deleted, which are used by other records.

User-defined integrity: It enforces some specific business rules that are defined by users. These rules are

different from entity, domain or referential integrity.

Difference between DBMS and RDBMS

The main differences between DBMS and RDBMS are given below:

S.No. DBMS RDBMS

1. DBMS applications store data as file. RDBMS applications store data in a tabular

form.

2. In DBMS, data is generally stored in either a

hierarchical form or a navigational form.

In RDBMS, the tables have an identifier

called primary key and the data values are

stored in the form of tables.

3. Normalization is not present in DBMS. Normalization is present in RDBMS.

4. DBMS does not apply any security with

regards to data manipulation.

RDBMS defines the integrity constraint for

the purpose of ACID (Atomocity,

Consistency, Isolation and Durability)

property.

5. DBMS uses file system to store data, so there

will be no relation between the tables.

in RDBMS, data values are stored in the

form of tables, so a relationship between

these data values will be stored in the form of

a table as well.

6. DBMS has to provide some uniform methods

to access the stored information.

RDBMS system supports a tabular structure

of the data and a relationship between them

to access the stored information.

7. DBMS does not support distributed

database.

RDBMS supports distributed database.

8. DBMS is meant to be for small organization

and deal with small data. it supports single

user.

RDBMS is designed to handle large

amount of data. it supports multiple users.

9. Examples of DBMS are file systems, xml etc. Example of RDBMS are mysql, postgre, sql

server, oracle etc.

DBMS vs. File System

Chapter 6

Introduction To SQL

Structure Query Language(SQL) is a database query language used for storing and managing data in

Relational DBMS. SQL was the first commercial language introduced for E.F Codd's Relational model

of database. Today almost all RDBMS(MySql, Oracle, Infomix, Sybase, MS Access) use SQL as the

standard database query language. SQL is used to perform all types of data operations in RDBMS.

OR

What is SQL?

o SQL stands for Structured Query Language.

o It is designed for managing data in a relational database management system (RDBMS).

o It is pronounced as S-Q-L or sometime See-Qwell.

o SQL is a database language, it is used for database creation, deletion, fetching rows, and

modifying rows, etc.

o SQL is based on relational algebra and tuple relational calculus.

All DBMS like MySQL, Oracle, MS Access, Sybase, Informix, Postgres, and SQL Server use SQL as

standard database language.

A Brief History of SQL

1970 − Dr. Edgar F. "Ted" Codd of IBM is known as the father of relational databases. He

described a relational model for databases.

1974 − Structured Query Language appeared.

1978 − IBM worked to develop Codd's ideas and released a product named System/R.

1986 − IBM developed the first prototype of relational database and standardized by ANSI. The

first relational database was released by Relational Software which later came to be known as

Oracle.

Why SQL is required?

SQL is required:

o To create new databases, tables and views

o To insert records in a database

o To update records in a database

o To delete records from a database

o To retrieve data from a database

What SQL does?

o With SQL, we can query our database in several ways, using English-like statements.

o With SQL, a user can access data from a relational database management system.

o It allows the user to describe the data.

o It allows the user to define the data in the database and manipulate it when needed.

o It allows the user to create and drop database and table.

o It allows the user to create a view, stored procedure, function in a database.

o It allows the user to set permission on tables, procedures, and views.

Advantages of SQL

There are the following advantages of SQL:

High speed

o Using the SQL queries, the user can quickly and efficiently retrieve a large amount of records

from a database.

No coding needed

o In the standard SQL, it is very easy to manage the database system. It doesn't require a substantial

amount of code to manage the database system.

Well defined standards

o Long established are used by the SQL databases that are being used by ISO and ANSI.

Portability

o SQL can be used in laptop, PCs, server and even some mobile phones.

Interactive language

o SQL is a domain language used to communicate with the database. It is also used to receive

answers to the complex questions in seconds.

Multiple data view o Using the SQL language, the users can make different views of the database structure.

Characteristics of SQL

o SQL is easy to learn.

o SQL is used to access data from relational database management systems.

o SQL can execute queries against the database.

o SQL is used to describe the data.

o SQL is used to define the data in the database and manipulate it when needed.

o SQL is used to create and drop the database and table.

o SQL is used to create a view, stored procedure, function in a database.

o SQL allows users to set permissions on tables, procedures, and views.

SQL Process

When you are executing an SQL command for any RDBMS, the system determines the best way to carry

out your request and SQL engine figures out how to interpret the task.

There are various components included in this process.

These components are −

Query Dispatcher

Optimization Engines

Classic Query Engine

SQL Query Engine, etc.

A classic query engine handles all the non-SQL queries, but a SQL query engine won't handle logical

files.

Following is a simple diagram showing the SQL Architecture −

SQL Data Types/ Literals

Data types are used to represent the nature of the data that can be stored in the database table. For

example, in a particular column of a table, if we want to store a string type of data then we will have to

declare a string data type of this column.

Data types mainly classified into three categories for every database.

o String Data types

o Numeric Data types

o Date and time Data types

In SQL, a literal is the same as a constant. We'll cover several types of literals - string, integer, decimal,

and datetime literals.

String Literals String literals are always surrounded by single quotes (').

For example:

'TechOnTheNet.com'

'This is a literal'

'XYZ'

'123'

Integer Literals

Integer literals can be either positive numbers or negative numbers, but do not contain decimals. If you do

not specify a sign, then a positive number is assumed. Here are some examples of valid integer literals:

536

+536

-536

Decimal Literals

Decimal literals can be either positive numbers or negative numbers and contain decimals. If you do not

specify a sign, then a positive number is assumed. Here are some examples of valid decimal literals:

24.7

+24.7

-24.7

Datetime Literals

Datetime literals are character representations of datetime values that are enclosed in single quotes. Here

are some examples of valid datetime literals:

'April 30, 2015'

'2015/04/30'

'2015/04/30 08:34:25'

SQL Command

https://www.studytonight.com/dbms/introduction-to-sql.php

SQL defines following ways to manipulate data stored in an RDBMS.

DDL: Data Definition Language

This includes changes to the structure of the table like creation of table, altering table, deleting a table etc.

All DDL commands are auto-committed. That means it saves all the changes permanently in the database.

Command Description

create to create new table or database

alter for alteration

truncate delete data from table

drop to drop a table

rename to rename a table

DML: Data Manipulation Language

DML commands are used for manipulating the data stored in the table and not the table itself.

DML commands are not auto-committed. It means changes are not permanent to database, they can be

rolled back.

Command Description

insert to insert a new row

update to update existing row

delete to delete a row

merge merging two rows or two tables

TCL: Transaction Control Language

These commands are to keep a check on other commands and their affect on the database. These

commands can annul changes made by other commands by rolling the data back to its original state. It can

also make any temporary change permanent.

Command Description

commit to permanently save

rollback to undo change

savepoint to save temporarily

DCL: Data Control Language

Data control language are the commands to grant and take back authority from any database user.

Command Description

grant grant permission of right

revoke take back permission.

DQL: Data Query Language

Data query language is used to fetch data from tables based on conditions that we can easily apply.

Command Description

select retrieve records from one or more table

SQL Operators

Operators manipulate individual data items called operands or arguments. Operators are represented by

special characters or by keywords. For example, the multiplication operator is represented by an asterisk

(*).

Unary And Binary Operators The two general classes of operators are:

unary: A unary operator operates on only one operand. A unary operator typically appears with its

operand in this format:

operator operand

binary: A binary operator operates on two operands. A binary operator appears with its operands

in this format:

operand1 operator operand2

Other operators with special formats accept more than two operands. If an operator is given a null

operand, the result is always null. The only operator that does not follow this rule is concatenation (||).

Operator precedence determines the grouping of terms in an expression. This affects how an expression

is evaluated. Certain operators have higher precedence than others; for example, the multiplication

operator has higher precedence than the addition operator.

For example, x = 7 + 3 * 2; here, x is assigned 13, not 20 because operator * has higher precedence than

+, so it first gets multiplied with 3*2 and then adds into 7.

Here, operators with the highest precedence appear at the top of the table, those with the lowest appear at

the bottom. Within an expression, higher precedence operators will be evaluated first.

The precedence of operators goes as follows: =, <, >, <=, >=, <>, !=, ~=, ^=, IS NULL, LIKE,

BETWEEN, IN.

Operator Operation

** exponentiation

+, - identity, negation

*, / multiplication, division

+, -, || addition, subtraction, concatenation

comparison

NOT logical negation

AND conjunction

OR inclusion

Aggregate functions in SQL

In database management an aggregate function is a function where the values of multiple rows are

grouped together as input on certain criteria to form a single value of more significant meaning.

Various Aggregate Functions

1) Count()

2) Sum()

3) Avg()

4) Min()

5) Max()

Now let us understand each Aggregate function with a example:

Id Name Salary

-----------------------

1 A 80

2 B 40

3 C 60

4 D 70

5 E 60

6 F Null

Count():

Count(*): Returns total number of records .i.e 6.

Count(salary): Return number of Non Null values over the column salary. i.e 5.

Count(Distinct Salary): Return number of distinct Non Null values over the column salary .i.e 4

Sum():

sum(salary): Sum all Non Null values of Column salary i.e., 310

sum(Distinct salary): Sum of all distinct Non-Null values i.e., 250.

Avg():

Avg(salary) = Sum(salary) / count(salary) = 310/5

Avg(Distinct salary) = sum(Distinct salary) / Count(Distinct Salary) = 250/4

Min():

Min(salary): Minimum value in the salary column except NULL i.e., 40.

Max(salary): Maximum value in the salary i.e., 80.

SQL Sub Query

A Subquery is a query within another SQL query and embedded within the WHERE clause.

Important Rule:

o A subquery can be placed in a number of SQL clauses like WHERE clause, FROM clause,

HAVING clause.

o You can use Subquery with SELECT, UPDATE, INSERT, DELETE statements along with the

operators like =, <, >, >=, <=, IN, BETWEEN, etc.

o A subquery is a query within another query. The outer query is known as the main query, and the

inner query is known as a subquery.

o Subqueries are on the right side of the comparison operator.

o A subquery is enclosed in parentheses.

o In the Subquery, ORDER BY command cannot be used. But GROUP BY command can be used

to perform the same function as ORDER BY command.

1. Subqueries with the Select Statement

SQL subqueries are most frequently used with the Select statement.

Syntax

SELECT column_name

FROM table_name

WHERE column_name expression operator

( SELECT column_name from table_name WHERE ... );

Example

Consider the EMPLOYEE table have the following records:

The subquery with a SELECT statement will be:

SELECT *

FROM EMPLOYEE

WHERE ID IN (SELECT ID

FROM EMPLOYEE

WHERE SALARY > 4500);

This would produce the following result:

2. Subqueries with the INSERT Statement

SQL subquery can also be used with the Insert statement. In the insert statement, data returned from the

subquery is used to insert into another table.

In the subquery, the selected data can be modified with any of the character, date functions.

Syntax:

INSERT INTO table_name (column1, column2, column3....)

SELECT *

FROM table_name

WHERE VALUE OPERATOR

Example

Consider a table EMPLOYEE_BKP with similar as EMPLOYEE.

Now use the following syntax to copy the complete EMPLOYEE table into the EMPLOYEE_BKP table.

INSERT INTO EMPLOYEE_BKP

SELECT * FROM EMPLOYEE

WHERE ID IN (SELECT ID

FROM EMPLOYEE);

3. Subqueries with the UPDATE Statement

The subquery of SQL can be used in conjunction with the Update statement. When a subquery is used

with the Update statement, then either single or multiple columns in a table can be updated.

Syntax

UPDATE table

SET column_name = new_value

WHERE VALUE OPERATOR

(SELECT COLUMN_NAME

FROM TABLE_NAME

WHERE condition);

Example

Let's assume we have an EMPLOYEE_BKP table available which is backup of EMPLOYEE table. The

given example updates the SALARY by .25 times in the EMPLOYEE table for all employee whose AGE

is greater than or equal to 29.

UPDATE EMPLOYEE

SET SALARY = SALARY * 0.25

WHERE AGE IN (SELECT AGE FROM CUSTOMERS_BKP

WHERE AGE >= 29);

This would impact three rows, and finally, the EMPLOYEE table would have the following records.

4. Subqueries with the DELETE Statement

The subquery of SQL can be used in conjunction with the Delete statement just like any other statements

mentioned above.

Syntax

DELETE FROM TABLE_NAME

WHERE VALUE OPERATOR

(SELECT COLUMN_NAME

FROM TABLE_NAME

WHERE condition);

Example

Let's assume we have an EMPLOYEE_BKP table available which is backup of EMPLOYEE table. The

given example deletes the records from the EMPLOYEE table for all EMPLOYEE whose AGE is greater

than or equal to 29.

DELETE FROM EMPLOYEE

WHERE AGE IN (SELECT AGE FROM EMPLOYEE_BKP

WHERE AGE >= 29 );

This would impact three rows, and finally, the EMPLOYEE table would have the following records.

The SQL UNION Operator

The UNION operator is used to combine the result-set of two or more SELECT statements.

Each SELECT statement within UNION must have the same number of columns

The columns must also have similar data types

The columns in each SELECT statement must also be in the same order

UNION Syntax

SELECT column_name(s) FROM table1

UNION

SELECT column_name(s) FROM table2;

UNION ALL Syntax

The UNION operator selects only distinct values by default. To allow duplicate values, use UNION ALL:

SELECT column_name(s) FROM table1

UNION ALL

SELECT column_name(s) FROM table2;

Note: The column names in the result-set are usually equal to the column names in the first SELECT

statement in the UNION.

SQL | Join (Inner, Left, Right and Full Joins)

A SQL Join statement is used to combine data or rows from two or more tables based on a common field

between them. Different types of Joins are:

INNER JOIN

LEFT JOIN

RIGHT JOIN

FULL JOIN

Example of JOIN

SELECT columnlist

FROM TableA

INNER JOIN

TableB ON join condition

Difference between Join and union in SQL